1. Field of the Invention
The present invention relates to an improved process for preparing methyl dithiocarbazinate. More specifically, the invention relates to an improved process for improving the yield and/or purity of methyl dithiocarbazinate.
2. Brief Description of the Prior Art
The prior art disclosures of the processes for preparing methyl dithiocarbazinate are limited by the paucity of information on reported experimental procedures and the excessive cost of commercial scale procedures. Laboratory reports present little information on how reaction temperatures would affect production of undesirable by-products, yield and purity, impact on waste treatment operations and the like. Commercial scale procedures are expensive because they involve the use of expensive reagents and/or expensive techniques such as separation of intermediate products and recrystallization of the final product.
Audrieth et al., J. Organic Chem., Vol. 19, pp. 733-741 (1954) discloses a process for preparing methyl dithiocarbazinate and converting it to thiocarbohydrazide. The process comprises a dropwise addition of 1.04 moles of carbon disulfide to 1.18 moles of potassium hydroxide and 1.1 moles of 85% hydrazine in 200 ml. of ethanol, in an ice bath. A heavy yellow oil containing potassium dithiocarbazinate separates during the addition. The resulting mixture is stirred and chilled and two volumes of ether are added to cause separation of more of the desired product (potassium dithiocarbazinate).
The oily layer is separated from the ether-alcohol layer and filtered to remove a small amount of an unidentified solid that is formed. The clear yellow solution is then dissolved in 300 ml. of water. The resulting solution is cooled in an ice bath and 1.05 moles of methyl iodide are added in approximately 10 batches. The reaction vessel is shaken and cooled alternately after each such addition until the methyl iodide is consumed. The reaction mixture is allowed to stand for several hours, being shaken occasionally to permit complete reaction. The methyl dithiocarbazinate is collected and recrystallized from ethanol.
Methyl dithiocarbazinate (24.4 gm., 0.2 mole) was dissolved in 200 ml. of absolute ethanol and 18 ml. (0.3 mole of hydrazine) of 85% hydrazine hydrate was added. The resulting solution was refluxed until no more solid thiocarbohydrazide precipitated (about 45 minutes). A small amount of 3-hydrazino-4-amino-5-mercapto-1,2,4-triazole that had formed was removed as follows. The reaction mixture was chilled, and the resulting solid product was collected and recrystallized from water acidified with a few drops of hydrochloric acid.
U.S. Pat. No. 4,696,938 discloses a process for preparing and using methyl hydrazinecarbodithioate as an intermediate in the preparation of 6-aryl-pyridine thiosemicarbazones. Methyl dithio-carbazinate is prepared as follows. Hydrazine hydrate (150 g) is added to a cooled (0.degree. C.) solution of potassium hydroxide in water (240 ml.) and 2-propanol (200 ml.). Pre-cooled carbon disulfide (182 ml.) is then added dropwise to the stirred reaction mixture, while maintaining an internal temperature below 10.degree. C. After the addition is complete, stirring is continued for a further one hour. Cooled methyl iodide (426 g) is added dropwise over 11/2 hours. The resulting white precipitate is collected by filtration and washed with cooled water. The crude product is recrystallized from methylene chloride.
To form 6-aryl-pyridine thiosemicarbazone, methyl dithiocarbazinate is reacted in a suitable solvent such as alcohol. The reaction product is treated with selenium dioxide in a suitable ethereal solvent such as tetrahydrofuran or 1,4-dioxane.
S. Losanitch, J. Chem. Soc., Vol. 119, pp. 763-765 (1921) discloses a process for preparing methyl dithiocarbazinate by first obtaining ammonium dithiocarbazinate and reacting it with methyl iodide. The ammonium dithiocarbazinate is obtained as follows. A solution of hydrazine hydrate in alcohol, containing a large excess of ammonia, is slowly treated with cooling with the corresponding quantity of carbon disulfide. The methyl dithiocarbazinate is formed by treating the ammonium salt in a dilute alcohol solution with methyl iodide.
Sandstrom et al, Arkiv For Kemi, 4(1952) 297, discloses a process for preparing ethyldithiocarbazinate. The process involves the separation of hydrazinium dithiocarbazinate from an ethanol-water mixture and the reaction of the hydrazinium dithiocarbazinate with ethyl bromide in an ethanol-water mixture.
U.S. Pat. No. 3,284,482 discloses a process for preparing chlorobenzyl esters of dithiocarbazinic acid as follows. To a solution comprising 85% hydrazine, 25% sodium hydroxide and 300 ml. of water is added carbon disulfide, dropwise at 10.degree. to 15.degree. C. over 20 minutes. External cooling is removed and the reaction mixture is stirred for an hour at 25.degree. to 30.degree. C. Then, trichlorobenzyl chloride is added in one portion to the reaction mixture which is stirred for 24 hours at 25.degree. to 30.degree. C. to produce the corresponding trichlorobenzyl dithiocarbazinate. The product is then extracted with ethyl ether. The ether solution is washed with water until it becomes neutral, is dried over sodium sulfate, and the ether is removed in vacuo.
British Patent Specification 1,274,521 discloses dithiocarbazinic ester derivatives by reacting dithiocarbazinic acid esters with an oxo compound. The dithiocarbazinic acid is prepared by reacting hydrazine hydrate with carbon disulfide in alcohol medium in the presence of potassium hydroxide, ammonia or excess hydrazine hydrate.
After isolation, the dithiocarbazinic acid salt is converted into an ester by an alkylating or aralkylating step. This step is carried out in water, a mixture of water and alcohol or in alcohol. Alternately, the ester can be prepared in a single reactor. The alkylating or aralkylating agent is added to the dithiocarbazinic acid salt solution prepared by the above method. The alkylating or aralkylating agents disclosed by the patent are: dimethyl sulfate, diethyl sulfate, allyl chloride, n-butyl iodide, n-octyl ester, n-dodecyl bromide, cetyl bromide, benzyl chloride, p-chlorobenzyl chloride, p-isopropylbenzyl bromide, p-n-butylbenzyl bromide, and alpha-methylbenzyl chloride.
As would be realized from the foregoing, there is a need for an economic process, i.e., a more facile and cost efficient process for preparing methyl dithiocarbazinate. By the present invention, there is provided such an improved process for preparing methyl dithiocarbazinate.